In the on-going search for subsurface hydrocarbons, methods have been developed for evaluating and interpreting the structure and characteristics of the earth's subsurface. The ability to accurately determine the volume of hydrocarbons present in subsurface reservoirs is particularly important.
The importance of such an ability is mostly driven by economic realities: if reservoirs can be defined more accurately, then better decisions can be made leading to higher drilling success and fewer development wells. The size of a reservoir and the total volume of hydrocarbons in place are critical for the decision to develop a field, estimating the terms of delivery, and designing the surface facilities. Under-designed facilities do not allow hydrocarbon extraction at optimal rates and overdesign adds unnecessary expenses.
For hydrocarbons to accumulate in the subsurface, both reservoir rocks and sealing rocks are needed. Reservoir rocks of high enough porosity are necessary to hold a sufficient quantity of hydrocarbons in their pore space. Impermeable sealing rocks are necessary to keep the hydrocarbons in place and prevent the hydrocarbons escaping to the surface. Various types of reservoir rocks can serve as reservoirs for hydrocarbons. Sandstones, also sands, are the most common. Seals are frequently provided by shales. Sand reservoirs do not usually consist of one thick slab of sand but rather of a number of sands separated by shales. The aggregate of sands and interbedded shales, the sand/shale package, is known in the industry as gross sand. The gross sand has a top and a base which are usually mapped on the basis of seismic data and, if available, well data. While it would be highly desirable to be able to map individual sands within the gross sand, the resolution of seismic data, in general, is not nearly high enough to make this possible. It is a more realistic objective, based on current technology, to map the total cummulative thickness of all the sands, the net-sand thickness, in the gross sand. This net-sand thickness, if known over the whole lateral extent of the gross sand, allows one to determine the total volume of sand and thus the size of the hydrocarbon container or the net-reservoir volume.
Accurately predicting the gross sand thickness variations of a reservoir and the net-sand thickness is essential for estimating the amount of hydorcarbon in place. Knowledge of the net-sand thickness as a function of location also allows one to properly place additional wells to optimally drain a reservoir.
The evaluation of reservoirs is typically achieved using a combination of seismic and well data. However, well data obtained from well logs of various types represent data samples from only a small fraction of a reservoir's volume surrounding a well. Three-dimensional seismic surveys provide seismic data samples over most of a reservoir's volume, including portions not sampled by wells. At best, however, the seismic data can provide only highly averaged information. Various methods have been tried to estimate the net-sand thickness from well data and seismic data. These methods usually rely on the fact that, particularly for large reflection angles, reflections from sand/shale interfaces are stronger than interfaces between different types of shale. Typical practice is to use a combination of well data and seismic data to predict the composition of reservoir rocks and lithology.
A recent approach involves generating one or more seismic attributes that are linked or physically related to the reservoir properties of interest. A seismic attribute is derived from a seismic trace. A seismic attribute is a quantitative derivative of a basic seismic measurement that may be extracted along a seismic trace, extracted along a horizon, or summed over a time window. Examples of seismic attributes are the peak amplitude of a seismic trace within a time window and the average magnitude of a seismic trace within a time window. The methodology involved in using seismic-guided estimates to characterize reservoir properties requires that the seismic data be reliably linked with the well data, both vertically and horizontally, correctly correlating seismic attributes to the reservoir properties of interest, and extrapolating the properties throughput the seismic volume.
Current methods are limited by the lack of a reliable physical relationship between the seismic attributes used and the geologic property of interest such as the net-sand thickness, arid are therefore prone to spurious correlations and results. Prior to the present invention, there was no existing methodology to directly measure net-sand thickness over a broad range of gross sand thicknesses. The contribution of the present invention resides in the discovery of a new method which directly determines net-sand thickness and net-reservoir volume over a broad range of gross sand thicknesses.